1. Field of the Invention
The present invention relates generally to ball valves, especially those in use in controlling the flow of fluids that tend to degrade process piping.
2. Discussion of Background
In many applications, especially in industrial processes, ball valves are used to control the flow of a fluid through piping. A ball valve is essentially a ball or sphere with a hole or port formed in it that is the same size as the pipe inner diameter. The ball is placed in a housing that is inserted in series with a pipe chase. The ball can be rotated to orient the port so that its axis is either aligned with the piping axis of the piping or perpendicular to it. When the port is aligned with the piping, the valve permits fluid flow; when perpendicular, flow is stopped.
Like many moving pans, the ball may be lubricated or the surface of the ball or the interior surface of its housing covered in a "slippery" plastic or synthetic material having a low coefficient of friction.
Sometimes, however, the environment of use of the valve is especially harsh. For example, if the fluid is corrosive or tends to form scale on metal surfaces, the ball may become difficult to turn within its housing, or the smooth tight fit of the ball against the inside of its housing may gradually be degraded to the point where the valve will start to leak.
One such harsh environment occurs in paper making. When wood pulp is processed, it is dissolved in a caustic chemical solution. The dissolved wood cellulose solution is called "green liquor." The green liquor will form scale on ball valves. As a result, ball valves used in piping that conducts green liquor require frequent descaling. When a process line is taken out of service for maintenance, that line is not productive and costs money.
Processing chlorine also is an inherently difficult fluid for controlling using ball valves. In addition to scale formation, chlorine trapped in the port of the ball valve when it is closed can explode. Therefore, a small vent hole is made in the ball to reduce the pressure on the trapped chlorine. This hole, typically 1/16 inch in diameter, is drilled so that, when the valve is closed, it vents upstream or downstream, but usually upstream.
Several valves have been designed for fluids that form scale on the valve's parts, including one specifically used in paper manufacturing, namely that disclosed by Zeigler, et al. in U.S. Pat. No. 3,191,906. Zeigler, et al. teach a valve where the moving part is not a ball, but a structure that is concave on the downstream side of the flow and convex on the upstream side when the valve is in the closed position. Bovee, et al., in U.S. Pat. No. 5,016,857, teach a similar configuration in a higher-flow-capacity valve, where, again, to close the valve, the convex side of the hollowed ball is rotated to face the upstream pipe.
Urban, U.S. Pat. No. 3,528,448, addresses the problem of scale formation in aluminum refining using a control element similar to those of Bovee, et al. and Zeigler, et al. His valve has scrapers to remove scale before it builds up. Like Bovee, et al.'s design, the convex side of Urban's valve faces upstream.
Kulisek's ball valve, described in U.S. Pat. No. 4,542,878, is directed to scale buildup. The surface of the ball of the valve that is exposed to the upstream side when the valve is closed is recessed.
Coatings in general for ball valves are not new. Smith, et al. describe a process in U.S. Pat. No. 4,531,273 for impregnating the surface of a ball valve, etc. with a polymer. Oliva-Bonino, in U.S. Pat. No. 3,951,380 discloses the use of TEFLON coatings on ball valves, and Yopp teaches a TEFLON seat for a ball valve in U.S. Pat. No. 3,814,381. See also the specification of Sanctuary in U.S. Pat. No. 3,348,805, for a description of another plastic resin coating, DELRIN, for ball valves.
However, there remains a need for a ball valve that can operate longer between scheduled maintenance because scaling does not affect its performance.